Multi-layer piezoelectric actuators constituted from piezoelectric layers and internal electrodes stacked alternately one on another have been known as an example of the multi-layer piezoelectric device. The multi-layer piezoelectric actuators can be divided into two categories: fired-at-once type and stacked type which has such a constitution as piezoelectric porcelain and internal electrode sheets are stacked one on another alternately. When the requirements to reduce the operating voltage and the manufacturing cost are taken into consideration, the multi-layer piezoelectric actuator of fired-at-once type is more advantageous for the reason of smaller layer thickness and higher durability.
FIG. 2 shows a multi-layer piezoelectric device of the prior art disclosed in Patent Document 1, which is constituted from a stack 200, that is formed by stacking piezoelectric layers 21 and internal electrodes 22 alternately one on another, and external electrodes 23 formed on a pair of side faces that oppose each other. While the stack 200 is formed by stacking the piezoelectric layers 21 and the internal electrodes 22 alternately one on another, the internal electrodes 22 are not formed over the entire principal surfaces of the piezoelectric layers 21, but have a so-called partial electrode structure. The piezoelectric layers are stacked in the so-called partial electrode structure such that the internal electrode 22 is placed in every other layer in a staggered manner so as to be exposed alternately at the left then at the right on different side faces of the stack 200. Inactive layers 24, 24 are stacked on both principal surfaces of the stack 200 in the direction of stacking. Then the external electrodes 23 are formed so that the internal electrodes 22 that are exposed on the pair of opposing side faces of the stack 200 are connected to each other, thereby connecting the internal electrodes 22 in every other layer.
In case the multi-layer piezoelectric device of the prior art is used as a piezoelectric actuator, lead wires are soldered onto the external electrodes 23 and operated by applying a predetermined voltage between the external electrodes 23. In recent years, since it is required to make a compact multi-layer piezoelectric device capable of achieving a large amount of displacement under a high pressure, it is in practice to carry out continuous operation over a long period of time with a higher electric field applied.
The multi-layer piezoelectric device is manufactured as follows. First, an internal electrode paste is printed in the pattern of a predetermined electrode structure as shown in FIG. 2 on a ceramic green sheet that contains the material of the piezoelectric layer 21, stacking a plurality of the green sheets coated with the internal electrode paste so as to form a multi-layer compact and firing the compact thereby to make the stack 200. Then the external electrodes 23 are formed on a pair of side faces of the stack 200 by firing, thereby to make the multi-layer piezoelectric device.
During manufacture of the stack 200 of the prior art, alkali metal may mix into the material. That is, the stock material mixed into the green sheet of the piezoelectric layer 21 and the binder may contain an alkali metal in the form of oxide, carbonate or nitrate, or may mix as an inevitable impurity. While a glass powder may be added to the stock material of the piezoelectric layer 21 for the purpose of improving the ease of sintering thereof, the glass powder often contains oxides of alkali metals. Moreover, the alkali metal may also mix into the material from crushing balls used in crushing the stock material to make the piezoelectric layer 21 or from the firing atmosphere through diffusion.
Halogen elements may also mix into the material. That is, the stock material used to make the piezoelectric layer 21 and the binder may include halogen elements in the form of fluoride, chloride, bromide, iodide or astatine compound, or mixing therein as an inevitable impurity. Also in the manufacturing process, use of water during crushing or storage of the stock material used to make the piezoelectric layer 21 over a long period of time may lead to mixing of halogen elements into the material. Moreover, halogen elements may also mix into the material from crushing balls used in crushing the stock material of the piezoelectric layer 21 or from the firing atmosphere through diffusion.
The alkali metals and halogen elements may also mix into the material in the form of compounds such as NaCl from human bodies.
The internal electrode 22 has been formed from an alloy of silver and palladium and, in order to fire the piezoelectric layers 21 and the internal electrodes 22 at the same time, composition of metals contained in the internal electrode 22 has been set to 70% by weight of silver and 30% by weight of palladium (refer to, for example, Patent Document 2).
The internal electrode is made of metal compound that contains silver-palladium alloy instead of pure silver because, when a voltage is applied between the pair of opposing electrodes that are made of silver without palladium content, the so-called silver migration occurs in which silver atoms migrate from the positive electrode to the negative electrode of the pair of the electrodes along the device surface. Silver migration occurs conspicuously particularly in an atmosphere of high temperature and high humidity.
During manufacture of the internal electrodes 22, alkali metal may mix into the internal electrodes 22. That is, the stock material of the internal electrodes 22 and the binder may include alkali metal in the form of oxide, carbonate or nitrate, or mix therein as an inevitable impurity. While a glass powder may be added to the stock material of the internal electrodes 22 for the purpose of improving the ease of sintering, the glass powder often contains oxides of alkali metals. Moreover, alkali metal may also mix into the material from crushing balls used in crushing the stock material of the internal electrodes 22 or from the firing atmosphere through diffusion.
Halogen elements may also mix into the internal electrodes 22. That is, the stock material used to make the internal electrodes 22 and the binder may include halogen elements in the form of fluoride, chloride, bromide, iodide or astatine compound, or mix into the material as an inevitable impurity. Also in the manufacturing process, storage of the stock material used of the internal electrodes 22 over a long period of time may lead to mixing of halogen elements into the material. Moreover, halogen elements may also mix into the material from the firing atmosphere through diffusion.
Both the alkali metals and halogen elements may also mix into the material in the form of compounds such as NaCl from human bodies.
During manufacture of the external electrodes 23 of the prior art, alkali metal may mix into the external electrodes 23. That is, the stock material of the external electrodes 23 and the binder may include alkali metal in the form of oxide, carbonate or nitrate, or mix therein as an inevitable impurity. While a glass powder may be added to the stock material of the external electrodes 23 for the purpose of improving the ease of sintering, the glass powder often contains oxides of alkali metals. Moreover, alkali metal may also mix into the material from crushing balls used in crushing the stock material of the external electrodes 23 or from the firing atmosphere through diffusion.
Halogen elements may also mix into the external electrodes 23. That is, the stock material used to make the external electrodes 23 and the binder may include halogen elements in the form of fluoride, chloride, bromide, iodide or astatine compound, or mix into the material as an inevitable impurity. Also in the manufacturing process, use of water in the mixing and crushing process or storage of the stock material used of the external electrodes 23 over a long period of time may lead to mixing of halogen elements into the material. Moreover, halogen elements may also mix into the material from crushing balls used in crushing the stock material of the external electrodes 23 or from the firing atmosphere through diffusion.
Both the alkali metals and halogen elements may also mix into the material in the form of compounds such as NaCl from human bodies.
When the multi-layer piezoelectric device is manufactured by firing the stack constituted from a plurality of the green sheets formed from the stock material of the piezoelectric layer 21 and the binder whereon the paste constituted from the stock material of the internal electrodes 22 and the binder is printed thereon, the alkali metal contained in the piezoelectric layer 21 and in the internal electrodes 22 may diffuse from a portion of higher concentration of alkali metal into a portion of lower concentration. Distance over which the diffusion propagates varies depending on the firing temperature, duration of firing and the ratio of concentrations. The halogen element contained in the piezoelectric layer 21 and in the internal electrodes 22 may also diffuse from a portion of higher concentration of halogen element into a portion of lower concentration. Distance over which the diffusion propagates varies depending on the firing temperature, duration of firing and the ratio of concentrations.
Also when the paste constituted from the stock material of the external electrodes 23 and the binder is printed and fired on the pair of side faces of the stack 200, alkali metals contained in the external electrodes 23 and in the piezoelectric layer 21 that is in contact with the external electrodes 23 may diffuse from a portion of higher concentration of alkali metal into a portion of lower concentration. Distance over which the diffusion propagates varies depending on the firing temperature, duration of firing and the ratio of concentrations. In addition, alkali metals contained in the external electrodes 23 and in the internal electrodes 22 that is in contact with the external electrodes 23 may diffuse from a portion of higher concentration of alkali metal into a portion of lower concentration. Distance over which the diffusion propagates varies depending on the firing temperature, duration of firing and the ratio of concentrations.
Similarly, halogen element contained in the external electrodes 23 and in the piezoelectric layer 21 that is in contact with the external electrodes 23 may also diffuse from a portion of higher concentration of the halogen element into a portion of lower concentration. Distance over which the diffusion propagates varies depending on the firing temperature, duration of firing and the ratio of concentrations. Similarly, halogen element contained in the external electrodes 23 and in the internal electrode 22 that is in contact with the external electrodes 23 may also diffuse from a portion of higher concentration of the halogen element into a portion of lower concentration. Distance over which the diffusion propagates varies depending on the firing temperature, duration of firing and the ratio of concentrations.
Alkali metals are very effective in assisting the sintering reaction of ceramics materials, and have long been used as the sintering assisting agent. However, excessive amount of alkali metal results in dielectric loss of high frequency energy. Therefore, it has been a common practice to decrease the content of alkali metal in order to decrease the dielectric loss in ceramic materials used in IC packages which suffer increasing transmission loss of signals when the dielectric loss of high frequency energy increases, and capacitors which suffer decreasing Q value and increasing heat generation when the dielectric loss of high frequency energy increases. In the multi-layer piezoelectric device, in contrast, the device is driven with a high DC voltage and operates at a low frequency of 1 kHz or less, unlike the applications described above. Therefore, high frequency dielectric characteristic is not of high priority for the multi-layer piezoelectric device. Since it has been required to form the piezoelectric layer 21 from a dense sintered material in order to achieve high insulation with regards to high voltage, alkali metal has been used as the sintering assisting agent.
Composite perovskite type compound containing PbTiO3—PbZrO3 (hereinafter abbreviated as PZT) as the main component has been used as ceramic or piezoelectric ceramic material. Most of the components of these materials are ceramic materials, which are formed by forming the stock material or calcined powder into a compact of predetermined shape and firing the compact at a high temperature. These piezoelectric ceramic materials have been made so as to provide various properties for such applications as actuator, ceramic filter and piezoelectric buzzer, by adjusting the proportions of the components. For example, a piezoelectric actuator consumes less power and generates less heat than the conventional electromagnetic actuator of the prior art made from a magnetic material with a coil wound around thereof, and has excellent properties such as fast response, larger amount of displacement, smaller size and smaller weight. However, PZT ceramics has drawbacks such as low 4-point bending strength that is about 100 MPa and susceptibility to cracks and breakage during machining.
Patent Document 3 discloses PZT piezoelectric ceramics that contains 0.01 to 0.3% by weight of Fe, 0.01 to 0.04% by weight of Al and 0.01 to 0.04% by weight of Si as auxiliary components for the purpose of suppressing cracks and breakage from occurring during machining with a grinder.
With the PZT piezoelectric ceramics disclosed in Patent Document 1, Al and Si added as the auxiliary components tend to form liquid phase in the sintering process, resulting in a glass phase that contains PbO—Al2O3—SiO2 in the grain boundary after the sintering process. Accordingly, a dense sintered material can be made with crystal grains grown therein at a temperature lower than that for sintering a piezoelectric ceramic material that does not include such auxiliary components. As a result, crack and breakage can be suppressed from occurring during machining with a grinder and the glass phase has higher rupture toughness than the perovskite type compound, thus increasing the rupture toughness of the sintered material.
Multi-layer piezoelectric actuators constituted from piezoelectric layers and internal electrodes stacked alternately one on another have been known as an example of the multi-layer piezoelectric device. The multi-layer piezoelectric actuators can be divided into two categories: fired-at-once type and stacked type in which piezoelectric porcelain and internal electrode sheets are stacked one on another alternately. When the requirements to reduce the operating voltage and the manufacturing cost are taken into consideration, the multi-layer piezoelectric actuator of fired-at-once type is more advantageous for the reason of smaller layer thickness and higher durability.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 61-133715    Patent Document 2: Japanese Unexamined Utility Model Publication (Kokai) No. 1-130568    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 14-220281